ZipSlip can be avoided by checking that the combined and resolved
path `StartsWith` the appropriate destination directory. Refine the
`StartsWith` sanitizer to:
* Consider expressions guarded by an appropriate StartsWith check to be
sanitized.
* Consider a StartsWith check to be inappropriate if it is checking the
result of `Path.Combine`, as that has not been appropriately resolved.
Tests have been updated to reflect this refinement.
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